1. Gluconeogenesis
Dr. Tarek A Salem

2. Objectives of this lecture
Definition of gluconeogenesis
The steps of gluconeogenesis
Site of occurrence and importance of gluconeogenesis.
The sources of carbon atoms used in gluconeogenesis

3. Definition
Glucneogenesis is making a new glucose from non-carbohydrate precursors
In other words:
Create new glucose from the products of its breakdown

4. Site of gluconeogenesis
Major site of gluconeogenesis: Liver (90%)
Secondary site: Kidney cortex and in small intestine under some conditions. (10 %)
It takes place in the mitochondria and cytoplasm.
The production of glucose is necessary for use as a fuel source by the brain, testes, erythrocytes, kidney medulla, lens and cornea of the eye and exercising muscle.

5. Overview
Synthesis of glucose from pyruvate utilizes many of the same enzymes as Glycolysis.
Three Glycolysis reactions have such a large negative ΔG that they are essentially irreversible
Hexokinase (or Glucokinase)
Phosphofructokinase
Pyruvate Kinase.
These steps must be bypassed in Gluconeogenesis.

6. First Bypass Reaction: Convervsion of Pyruvate to Phosphoenolpyruvate
Enzymes involved:
Pyruvate carboxylase
PEP carboxykinase

7. In Mitochondria
Pyruvate carboxylase (PC) presents in the mitochondria of liver and kidney but absent in muscle
ATP, biotin, Mn++ and CO2 are required.

8. Transport of Oxaloacetate into cytosol as Malate

9. In cytosol

10. Summary of the first bypass

11. The overall equation for this set of bypass reactions
Pyruvate + ATP + GTP + HCO3-
phosphoenolpyruvate + ADP + GDP + Pi + H+ + CO2
Thus the synthesis of one molecule of PEP requires an investment of 1 ATP and 1 GTP.
Note: when either pyruvate or the ATP/ADP ratio is high, the reaction is pushed toward the right (i.e., in the direction of biosynthesis).

12. Second Bypass Reaction: Conversion of Fructose 1,6- bisphosphate to Fructose 6-phosphate
The second glycolytic reaction (phosphorylation of fructose 6-phosphate by PFK) is irreversible.
Hence, for gluconeogenesis fructose 6-phosphate must be generated from fructose 1,6-bisphosphate by a different enzyme: Fructose 1,6-bisphosphatase.
Fructose 1,6-bisphosphatse presents in liver and kidney.
This reaction is also irreversible.
Fructose 1,6-bisphosphate + H2O fructose 6-phosphate + Pi

13. Phosphofructokinase (In Glycolysis):
fructose-6-P + ATP  fructose-1,6-bisP + ADP
Fructose-1,6-bisphosphatase (In Gluconeogenesis):
fructose-1,6-bisP + H2O  fructose-6-P + Pi

14. Third Bypass Reaction: Glucose 6-phosphate to Glucose
Because the hexokinase reaction is irreversible, the final reaction of gluconeogenesis is catalyzed by Glucose 6-phosphatase.
Glucose 6-phosphate + H2O glucose + Pi
Glucose 6-phosphatase is present in the liver, kidney and small intestine but absent in brain and muscle. Thus, glucose produced by gluconeogenesis in the liver, is delivered by the bloodstream to brain and muscle.

15. Hexokinase or Glucokinase (In Glycolysis):
glucose + ATP  glucose-6-phosphate + ADP
Glucose-6-Phosphatase (In Gluconeogenesis):
glucose-6-phosphate + H2O  glucose + Pi

16. Summary of Gluconeogenesis Pathway:
Gluconeogenesis enzyme names in red.
Glycolysis enzyme names in blue.

18. Carbon sources for gluconeogenesis
Glycerol: is released during hydrolysis of triacylglycerols in adipose tissue and is delivered by the blood to the liver.
Glycerol kinase
Glycerol Glycerol 3-P
G3P dehydrogenase
Glycerol 3-P DHAP
DHAP is converted into glyceraldehyde 3-P

19. Carbon sources for gluconeogenesis
Lactate (Lactic acid): In vigorous skeletal muscle activity, large amount of lactic acid produced pass to liver through blood stream converted into pyruvic and lastly to glucose reach muscle again through blood stream to provide energy. (This called Cori cycle).

20. Lactate produced from pyruvate passes via the blood to the liver, where it may be converted to glucose.
The glucose may travel back to the muscle to fuel Glycolysis.

21. Carbon sources for gluconeogenesis
Propionic acid: product of odd number fatty acid degradation.
It is converted into succinyl CoA which converted into oxaloactic acid that can form glucose.

22. Carbon sources for gluconeogenesis
Glucogenic amino acids: amino acids by deamination can be converted into keto acids as pyruvic, α-ketoglutaric and oxaloacetic acid.
Proteins are considered as one of the main sources of blood glucose especially after 18 hr due to depletion of liver glycogen.

23. Carbon sources for gluconeogenesis
Glucose-alanine cycle: During starvation, alanine is formed from protein catabolism.
Alanine is converted into pyruvic acid in liver which can give glucose.

24. Acetyl CoA can not produce glucose
Acetyl CoA cannot give rise to a net synthesis of glucose. This is due to the irreversible nature of the pyruvate dehydrogenase reaction, which converts pyruvate to acetyl CoA.
Pyruvate dehydrogenase
Pyruvate acetyl CoA + CO2
NAD NADH+H+

25. Importance of gluconeogenesis
1- Maintenance of blood glucose during starvation, fasting and prolonged exercise. 2- Removal of lactic acid. 3- Removal of glycerol produced by lipolysis.

26. Regulation of gluconeogenesis
In liver: ATP PK PC glycolysis is inhibited and gluconeogenesis is activited
During starvation, the priority is to conserve blood glucose for the brain and muscle. Thus, under these conditions, PK in the liver is switched off. This occurs because the hormone glucagon is secreted into the blood­stream and activates a cAMP cascade that leads to the phosphorylation and inhibition of this enzyme.

27. Regulation of gluconeogenesis
In liver: AMP PK PC glycolysis is activated and gluconeogenesis is stopped.
In case of Acetyl CoA PK PC
When acetyl CoA is abundant, gluconeogenesis is activated